Iron oxides with a higher degree of refining

ABSTRACT

Iron oxides are upgraded by calcining at from 700 to 1200° C.

DESCRIPTION

The present invention relates to a process for upgrading iron oxide,which comprises calcining iron oxide at not less than 700° C.

EP-A-1 027 928 describes a catalyst, especially for dehydrogenation ofethylbenzene to styrene, which is prepared using an iron oxide obtainedby spray roasting an iron salt solution, especially hydrochloric acidiron solutions (Ruthner process). The disadvantage of such iron oxidesis their high residual chloride content.

EP-A-797 481 discloses iron oxides as a starting material for catalysts,especially for dehydrogenation of ethylbenzene to styrene, which arerestructured by mixing with a further metal compound and subsequentcalcination and which have very small BET surface areas. Thedisadvantage of such a restructuring is the contamination of the ironoxide by the metal compound added.

JP-A-61-72601 discloses a fluidized bed process for cracking heavyhydrocarbons into lighter hydrocarbons using a pulverulent catalystprepared by slurrying up an iron oxide powder with water, spray dryingand finally calcining at temperatures between 1200 and 1600° C.Disadvantages of this process are the immense cost and inconvenience andthe high calcination temperatures.

EP-A-827 488 describes a process for reducing the residual chloridecontent in iron oxides, especially in iron oxides generated by sprayroasting hydrochloric acid pickling wastes, by mixing the iron oxidewith a hydrated metal compound and subsequent calcination. Thedisadvantage of this process is its cost and inconvenience.

U.S. Pat. No. 4,134,858 discloses roasting iron oxide at 800° C. beforeusing it to prepare styrene catalysts. However, any residual chloridecontent in the iron oxide cannot be sufficiently lowered by this method.

U.S. Pat. No. 2,414,585 discloses precalcining iron oxide for preparingdehydrogenation catalysts. The iron oxide obtained is said to have a BETsurface area of<8 m²/g and preferably of about 4 m^(2/)g. Such catalystsleave a lot to be desired.

It is an object of the present invention to remedy the aforementioneddisadvantages.

We have found that this object is achieved by a novel and improvedprocess for upgrading iron oxide, which comprises calcining iron oxidesat from 700 to 2500° C. The invention further provides novel iron oxidesand their use as catalysts, and for the preparation of catalysts,especially for the preparation of catalysts for dehydrogenation ofethylbenzene to styrene.

The process of the invention can be carried out as follows:

The upgrading of the present invention may be applied to any iron oxide,but is preferably applied to iron oxides generated by working uphydrochloric acid waste liquids from steel pickling, for example,especially iron oxides generated by spray roasting hydrochloric acidpickling wastes (Ruthner process).

The iron oxide may be subjected to a batch operated or preferablycontinuous calcination at from 700 to 1200° C., preferably at from 840to 1150° C., particularly preferably at from 850 to 1100° C., especiallyat from 860 to 1000° C., or generally from 0.1 to 24 h, preferably from0.25 to 10 h, particularly preferably from 0.3 to 5 h, especially from0.5 to 1.5 hr, without pretreatment, i.e., for example withoutmechanical pretreatment, and preferably dry, i.e., without priortreatment with water, an acid or base or some other material. Usefulcalcination apparatus includes all known ovens. The calcination can becarried out batchwise, for example in muffle furnaces, or continuously,for example in rotary tube ovens or in belt calciners. Preference isgiven to continuous processes. The calcination can be carried out atjust one temperature or in stages at various temperatures or in the formof a continuous temperature ramp. When the calcination is carried out inrotary tubes, the rotary tube should be equipped with tappers whichprevent sticking of the iron oxide to the wall of the rotary tube andensure continuous transportation of the iron oxide. Advantageously, thecalcination is carried out in smooth rotary tubes without internalfitments, and the residence time can be adjusted via the speed ofrotation, the feed speed and the inclination of the rotary tube. Thecalcination is further advantageously carried out under a gas stream,for example nitrogen or air, in order that chlorine compounds beingreleased may be expelled and advantageously removed in a downstreamoff-gas scrub. The chloride content is advantageously reducible in astationary bed, i.e., for example in the course of calcination in amuffle furnace or on a belt calciner. When a calcination is carried outin a moving bed, for example in a rotary tube, comparatively somewhathigher temperatures can be required to reduce the chloride content thanin a stationary bed, and this can lead to a comparatively furtherreduced BET surface area. Depending on the preferred ratio of chloridecontent and BET surface area, it can therefore be advantageous tooperate selectively with a stationary bed or with a moving bed.

However, small amounts of water, acids, bases or organic compounds maybe added, provided this does not adversely affect the properties of theupgraded iron oxide compared to a dry upgrading process. Preference isgiven to calcining commercially available iron oxide without anypretreatment whatever.

Useful iron oxides for the upgrading according to the invention includeall iron oxides, regardless of how obtained. Natural, preferablyindustrially produced and also commercially available iron oxides aresuitable, especially iron oxides generated by working up hydrochloricacid pickling wastes. These iron oxides maybe contain impurities, forexample a residual chloride content and/or compounds of titanium,manganese, aluminum, chromium, phosphorus, zinc, copper, molybdenum,tungsten, silicon, nickel, magnesium, potassium, sodium, cobalt,vanadium, zirconium, niobium, sulfur, lanthanum, lead, tin and/orcalcium. Of particular suitability are iron oxides which are generatedby spray roasting hydrochloric acid pickling wastes in the steelindustry and are present as Fe₂O₃ having a residual chloride content inthe range from 0 to 10 000 ppm, preferably in the range from 50 to 5000ppm and particularly preferably in the range from 500 to 2000 ppm,usually in the hematite crystal form and in a BET surface area oftypically from 3 to 5 m²/g.

Iron oxides upgraded according to the invention generally have aresidual chloride content of less than 400 ppm, preferably less than 300ppm and particularly preferably less than 250 ppm, especially less than200 ppm. The average particle size, determined by laser diffraction ashereinbelow described, is generally more than 5 μm, i.e., from 5.1 to200 μm, preferably from 8 to 100 μm, particularly preferably from 10 to80 μm and very particularly preferably from 12 to 30 μm, and the finesfraction having particle sizes of less than 1 μm is generally less than15% by weight, preferably less than 10% by weight, particularlypreferably less than 5% by weight. The BET surface area of the ironoxides treated according to the invention is generally in the range from0.4 to 5 m²/g, preferably in the range from 0.4 to 3.5 m²/g,particularly preferably in the range from 0.5 to 3 m²/g and especiallyin the range from 0.6 to 2.5 m²/g, very particularly preferably in therange from 0.7 to 2 m²/g. The iron oxides treated according to theinvention generally have a hematite structure. They are useful for awhole series of industrial applications such as pharmaceuticals,cosmetics, magnetic tape coatings, chemical reactions, catalysts or forpreparing catalysts, especially for preparing catalysts fordehydrogenation of ethylbenzene to styrene.

The industrial production of styrene by dehydrogenation of ehtylbenzenecan be effected by isothermal processes or by adiabatic processes. Theisothermal process is generally operated at from 450 to 700° C.,preferably from 520 to 650° C., in the gas phase with addition of watervapor at from 0.1 to 5 bar, preferably from 0.2 to 2 bar, particularlypreferably from 0.3 to 1 bar, especially from 0.4 to 0.9 bar. Theadiabatic process is generally operated at from 450 to 700° C.,preferably from 520 to 650° C., in the gas phase with addition of watervapor at from 0.1 to 2 bar, preferably from 0.2 to 1 bar, particularlypreferably from 0.3 to 0.9 bar, especially from 0.4 to 0.8 bar.Catalysts for the dehydrogenation of ethylbenzene to styrene can beregenerated by means of water vapor.

Catalysts for the dehydrogenation of ethylbenzene to styrene generallycontain iron oxide and an alkali metal compound, for example potassiumoxide. Such catalysts generally further contain a number of promoters.Promoters described include for example compounds of calcium, magnesium,cerium, molybdenum, tungsten, chromium and titanium. The catalysts maybe prepared using compounds of the promoters that will be present in theready-produced catalyst or compounds which during the production processconvert into compounds that are present in the ready-produced catalyst.The materials used may also include assistants to improve theprocessibility, the mechanical strength or the pore structure. Examplesof such assistants include potato starch, cellulose, stearic acid,graphite or Portland cement. The materials used can be mixed directly ina mixer, kneader or preferably a muller. They can also be slurried upinto a sprayable mix and be spray dried to form a powder. The materialsused are preferably processed in a muller or kneader in the presence ofwater to form an extrudable mass. The extrudable mass is subsequentlyextruded, dried and calcined. Preferred extrudates are from 2 to 10 mmin diameter. The cross section of the extrudates may be round or someother shape. Particular preference is given to extrudates having arotationally symmetrical cross section, especially 3 mm in diameter, andalso extrudates having a star-shaped cross section or a toothed-wheelcross section, especially 4.5 or 6 mm in diameter. The extrudates can bebroken or cut. As an alternative to an extrusion, the catalysts may alsobe shaped by tableting. Generally, the catalysts prepared according tothe invention have larger mean pore radii and a smaller BET surface areathan catalysts otherwise prepared similarly, but from iron oxide notupgraded according to the invention.

Catalysts prepared using the iron oxide upgraded according to theinvention instead of conventional, prior art iron oxide exhibit improvedactivity and selectivity. The fraction of iron oxide pretreatedaccording to the invention as a percentage of all the iron oxide presentin the catalyst should be not less than 30% by weight, preferably notless than 60% by weight and very particularly preferably not less than90% by weight. It is especially preferred for the fraction of iron oxideupgraded according to the invention as a percentage of the total ironoxide content to be 100% by weight. Catalysts for the dehydrogenation ofethylbenzene to styrene which have been prepared using iron oxideupgraded according to the invention are useful in all processes andprocess variants. They are particularly useful at steam/ethylbenzene(S/EB) ratios of from 0.6 to 2.5 kg/kg. They are very particularlyuseful at steam/ethylbenzene ratios of from 0.9 to 1.5 kg/kg. Thecatalysts prepared according to the invention are notable for a lowchloride content of less than 500 ppm, typically less than 300 ppm,particularly less than 200 ppm and an average pore diameter of from 0.3to 3 μm, typically from 0.5 to 1.5 μm.

EXAMPLES

Inventive example 1 and comparative example A utilized the HP (HōschPremium) iron oxide from Thyssen-Krupp, which is produced according tothe Ruthner process by spray roasting hydrochloric acid iron solutions.The residual chloride content was in all cases determinedcoulometrically. The particle size of the iron oxide was determinedusing a Mastersizer S from Malvern (lense: 300 RFmm, measuring range0.05 to 880 μm). The MS17 model was used. It is a sample feeder fordispersion in an aqueous medium having a built-in paddle stirrer, anintegrated ultrasonic probe and a circulation pump. Prior to themeasurement, the integrated ultrasonic bath (100% setting) was startedup and, following a dispersion time of 5 min, the measurement wascarried out under continuing ultrasonication. Specific BET surface areaswere determined according to DIN 66133 and pore volumes and average poreradii according to DIN 66131.

Inventive Example 1

2 kg of HP type iron oxide from Thyssen-Krupp were heated to 900° C. ina muffle furnace, left in the oven at this temperature for 1 h andsubsequently allowed to cool down with the oven.

Inventive Example 2

Inventive example 1 was repeated at 800° C.

Inventive Example 3

Inventive example 1 was repeated at 850° C.

Inventive Example 4

Inventive example 1 was repeated at 950° C.

Inventive Example 5

20 g of HP type iron oxide from Thyssen-Krupp were heated to 900° C. ina quartz glass rotary tube under an air stream, maintained therein at900° C. for 1 h and then allowed to cool down therein.

Inventive Example 6

Inventive example 5 was repeated, except that the iron oxide wasmaintained at 900° C. for 2 h.

Inventive Example 7

The iron oxide was continuously calcined under an air stream in a rotarytube. The rotary tube was equipped with three tappers. The walltemperature of the rotary tube was 970° C. and the residence time of theiron oxide was about one hour.

The physical properties of the inventively pretreated iron oxides ofinventive examples 1 to 7 are summarized in table 1 and compared thereinwith those of the nonupgraded iron oxide.

Inventive Example 8

A spray slurry prepared by suspending 420 g of potassium carbonate(potash), 516 g of cerium carbonate hydrate (40% by weight ceriumcontent), 74 g of ammonium heptamolybdate, 70 g of calcium hydroxide(white chalk hydrate), 55 g of magnesite and 1880 g of the iron oxideupgraded according to inventive example 1 in 4.5 liters of water wassprayed to form a powder which was pasted up with sufficient water(about 500 ml) in the presence of starch in a kneader to form anextrudable mass which was extruded into strands 3 mm in diameter. Thestrands were then dried at 120° C., broken to a length of about 0.8 mmand finally calcined in a rotary tube at 875° C. for 1 h.

Inventive Example 9

415 ml of a catalyst of inventive example 2 were tested in an externallyheated tubular reactor 3 cm in internal diameter under the conditionsreported in table 3.

Comparative Example A

A catalyst was prepared similarly to inventive example 8 except that theupgraded iron oxide of inventive example 1 was replaced by nonupgradediron oxide (HP type from Thyssen-Krupp).

Comparative Example B

415 ml of the catalyst of comparative example A were tested similarly toinventive example 3 in the same reactor under the same conditions. Theresults are summarized in table 3.

TABLE 1 Properties of a commercially available iron oxide prepared bythe Ruthner process (type HP from Thyssen-Krupp) and of iron oxidesprepared therefrom by the upgrading according to the invention frominventive examples 1 to 7. Fines less BET Residual Average than surfacechloride particle size 1 mm area Iron oxide [ppm] [μm] [% by weight][m²/g] Untreated HP type 1400 11 15 4.3 (Thyssen-Krupp) Inventiveexample 1 66 19 2 1.4 Inventive example 2 240 15 2.5 2.1 Inventiveexample 3 110 18 1.7 1.8 Inventive example 4 19 22 1 1.1 Inventiveexample 5 230 — — 1.6 Inventive example 6 190 — — 0.7 Inventive example7 190 13 1.9 0.9

TABLE 2 Comparison of physical properties of the catalyst of inventiveexample 2, prepared using an iron oxide upgraded according to theinvention, and of the catalyst of comparative example A, preparedaccording to the prior art. BET Average Cut surface area Pore volumepore diameter resistance [m²/g] [ml/g] [μm] [N] Inventive 1.3 0.27 1.2864 example 8 Comparative 2.9 0.25 0.39 51 example A

TABLE 3 Comparison of conversion and selectivity for dehydrogenation ofethylbenzene to styrene using an inventive catalyst (inventive example3) and a prior art catalyst (comparative example B). InventiveComparative Pres- example 9 example B sure Steam/EB Tempera- EBconversion EB conversion LHSV abs. ratio ture (styrene (styrene [h⁻¹][bar] [kg/kg] [° C.] selectivity) selectivity) 0.85 0.5 1.4 590 70.2%(96.2%) 68.3% (96.1%) 0.85 0.5 1.4 570 54.5% (97.2%) 53.9% (97.2%) 0.450.5 1.5 550 46.7% (97.4%) 45.7% (97.2%) 0.45 0.4 1.1 550 46.2% (97.9%)41.5% (97.9%)

1. A process for upgrading an iron oxide, comprising: working up hydrochloric acid pickling waste to generate iron oxide, and calcining the generated iron oxide at a temperature from 840 to 1150° C., optionally under nitrogen or air stream without the addition of other substances, wherein calcination is conducted without mechanical pretreatment, without prior treatment with water and without prior treatment with an acid or a base.
 2. A process as claimed in claim 1, wherein said iron oxide is calcined at from 850 to 1000° C.
 3. A process as claimed in claim 1, wherein the calcining is effected using a rotary tube equipped with tappers.
 4. The process as claimed in claim 1, wherein the upgraded iron oxide has a residual chloride content of less than 400 ppm.
 5. The process as claimed in claim 1, wherein the upgraded iron oxide has a chloride content of less than 200 ppm.
 6. The process as claimed in claim 1, wherein the upgraded iron oxide has an average particle size of more than 5 μm.
 7. A process for preparing a catalyst comprising: working up hydrochloric acid pickling waste to generate iron oxide, calcining the generated iron oxide at a temperature from 840 to 1150° C., optionally under nitrogen or air stream without the addition of other substances, wherein calcination is conducted without mechanical pretreatment, without prior treatment with water and without prior treatment with an acid or a base, and wherein the iron oxide has a residual chloride content of less than 400 ppm; and processing the calcined iron oxide to obtain the catalyst.
 8. A dehydrogenation process comprising: generating styrene by dehydrogenating ethylbenzene at a temperature of from 450 to 700° C. and a pressure of from 0.1 to 5 bars with a catalyst, wherein said catalyst is made by a process comprising: working up hydrochloric acid pickling waste to generate iron oxide, calcining the generated iron oxide at a temperature from 840 to 1150° C., optionally under nitrogen or air stream without the addition of other substances, wherein calcination is conducted without mechanical pretreatment, without prior treatment with water and without prior treatment with an acid or a base, and wherein the iron oxide has a residual chloride content of less than 400 ppm; and processing the calcined iron oxide to obtain the catalyst. 